Only a handful of cases stump America's most experienced investigators. This is one of them.
In 1971, a well-groomed businessman-type boarded a Boeing 727 in Portland, Oregon. It was bound for Seattle, Washington, but the passenger had other plans.
Shortly after the Northwest Orient Airlines jet took off, the man calling himself Dan Cooper gestured to a flight attendant before handing her a napkin with a note scrawled in capital letters. On it were demands for $200,000 cash, four parachutes and his freedom.
The man, believed to be in his early 40s, told the woman he had a bomb. He showed her the inside of his briefcase where eight red cylinders were attached to a series of wires.
Cooper's demands were met. He jumped from the plane somewhere over Reno, Nevada, and was never seen again.
This week, the FBI reluctantly announced the investigation had ended with no new leads.
"The FBI has redirected resources allocated to the D.B. Cooper case to focus on other investigative priorities," the bureau said.
Everything about the midair hijacking suggested Dan Cooper, otherwise known as "D.B.", was no amateur.
He was calm when he boarded the plane and took his seat. He ordered a bourbon and soda, smoked a filtered cigarette and put on dark sunglasses before making his move.
He had planned the hijacking intricately. He targeted Florence Schaffner, the flight attendant closest to where he was sitting. After passing her the note, she slipped it into her purse. Cooper reportedly told her, "Miss, you'd better look at that note."
He asked her to sit next to him where he detailed his demands. They were as follows: $200,000 cash, four parachutes, a fuel truck standing by at a Seattle airport and a second flight towards Mexico City.
FBI agents rushed to meet his requests. They secured money from various Seattle banks and provided parachutes from a local skydiving school.
When the plane landed, he let passengers and crew disembark.
When the plane was back in the air, he instructed the pilot to fly as slowly as possible without stalling. He opened a rear exit door and jumped into the darkness shortly after 7.40pm.
Police surrounded the plane when it landed at Reno Airport but the suspect was gone, along with the money.
One theory about his disappearance involves him dying in the fall, but no body was ever recovered. Another suggests Cooper was a former paratrooper.
Others have said he was John List, who killed his entire family the year the plane was hijacked, or Robert Rackstraw, a Vietnam vet who now lives a comfortable life on a 14m yacht.
'We were sure it was Dad'
Lisa Lepsy's father Richard disappeared in 1971. He has never been found. Two years after Cooper's midair stunt, the Lepsy family were watching news coverage of it.
"We were sitting on the couch watching Walter Cronkite," Lisa told local TV station WZZM.
"When the composite sketch of D.B. Cooper came on the TV screen, everyone looked at each other and said, 'That's Dad!'
"We were stunned because the resemblance was unbelievable, and my brothers and I were all sure that was our dad."
The family spent decades pursuing links between the two cases but authorities never took them seriously. In 1982, they wrote to the Mexican Consulate, just in case Lepsy had fled there.
Others have claimed Dan Cooper was a family member, too. Among them is Marla Cooper who, as an 8-year-old, recalled her uncle Lynn Doyle Cooper attending a Thanksgiving dinner in 1971 suffering serious injuries.
Lynn told the family he had been in a car crash but, in 2011, Marla spoke out. She told police her uncle was fascinated with a comic book character named Dan Cooper. She said that could help explain his alias.
The FBI investigated, telling her Lynn would not be ruled out as a suspect and declaring her story "significant and credible". That's as far as it got.
John List, who died in 2008 aged 82, was at one point considered a primary suspect. List was responsible for the 1971 murders of his wife, his mother and his three children.
He was on for the run for 18 years before he was apprehended in 1989 and sentenced to five consecutive terms of life imprisonment.
An FBI spokesman said following List's arrest: "John List is one of any number of people suspected in the D.B. Cooper case. He will be investigated until he is eliminated."
Did he even survive the jump?
The FBI's statement on Monday made it clear they believe Cooper may have died in the jump.
"Perhaps Cooper didn't survive his jump from the plane," the bureau's statement read.
"After all, the parachute he used couldn't be steered, his clothing and footwear were unsuitable for a rough landing, and he had jumped into a wooded area at night -- a dangerous proposition for a seasoned pro, which evidence suggests Cooper was not.
"This theory was given an added boost in 1980 when a young boy found a rotting package full of $20 bills (US$5800 in all) that matched the ransom money serial numbers."
But the absence of a body kept the mystery alive. Last week, shortly before the FBI released its statement, the History Channel aired a documentary driven by exclusive access to former FBI agents.
The documentary named a new suspect: Robert Rackstraw, a 72-year-old man who allegedly tried to fake his own death seven years after D.B. Cooper went missing.
A team of retired investigators said Rackstraw was living in San Diego Bay, on a 14m cruiser named Poverty Sucks.
Rackstraw's past is littered with criminal activity. The San Jose Mercury News reports he was arrested on suspicion of possession of explosives and writing $75,000 worth of bad cheques. He was found in Iran in 1978 and deported to New York City.
Rackstraw was a pilot in his younger years and trained extensively as a paratrooper. However, he denies the claims and so does his lawyer, Dennis Roberts.
"He's not D.B. Cooper," Roberts told the Mercury News. "Everything I've heard is that D.B. Cooper died, and Rackstraw is alive."
But Rackstraw's second wife, Linda McGarity, 67, told People she believes Rackstraw and Cooper are the same person.
"I think there's a good likelihood it's him," she said. "I believe it because of all the evidence and all the little things that I can look at from way back that just make me just go, 'Oh yeah'. All the pieces just sort of come together."
The hunt for D.B. Cooper will continue despite the FBI closing the official file. As the years go by and the mystery deepens, Cooper's cult status grows ever larger.
In 1975, James Cain penned a fictional novel titled Rainbow's End about Cooper's disappearance. It was followed by multiple fictional accounts of what happened on the day he hijacked the 727.
In 2000, Todd Snider wrote and performed the song D.B. Cooper. There are 10 other tributes to Cooper's exploits, including The Final Flight of D.B. Cooper by Victims of Circumstance and The Ballad of D.B. Cooper by Chuck Brodsky.
The FBI says it has "considered more than 800 suspects and eliminated all but two dozen from consideration".